For fill level measurement of fluids or bulk goods in containers, measuring devices are used, which determine fill level using a travel-time method. Examples of such devices include radar measuring devices. Such measuring devices use a microwave signal, which is radiated, for example via a horn antenna, toward the medium to be detected. The signal reflected on the medium is evaluated in the measuring device by an electronics unit. Determinable from the travel time of the signal is the distance to the medium and therefrom the fill level of the medium. Known are pulse radar systems and continuously radiating radar systems. In the case of so-called TDR probes, the signal is guided along a signal line protruding into the medium. An additional measuring device, which works according to the travel time principle, is an ultrasonic measuring device, which ascertains the distance to the medium by means of an ultrasonic signal.
In the case of fill level measurement by means of a travel-time method, measurement inaccuracies can occur, when, above the process medium, whose fill level is to be detected, instead of air, vacuum or some other medium of very low relative dielectric constant, another medium is present, for example, a gas phase, since this affects the travel time of the signal. It would be, consequently, advantageous to know the medium located between the measuring device and the process medium to be detected. At least the dielectric constant of the medium should be known.
Known from (laid open application) Offenlegungsschrift DE 102006045711 A1 is a method for distinguishing different fill materials by means of microwave measurement. For this, microwaves are radiated toward the medium to be examined and evaluated by a spatially remote sensor.